Gastrinomas are rare tumors of the pancreas and duodenum that derive their name from their ability to produce large amounts of gastrin, and patients usually come to medical attention becuase of the severe peptic ulcer disease that the gastrin causes. 30% of patients inherit the tendency to form gastrinomas as part of the Multiple Endocrine Neoplasia Type 1 (MEN1) syndrome, but even sporadic tumors often show alterations of the MEN1 gene on Chromosome 11. 8 tumors have been examined by high density quantitative genotyping at 400 loci evenly distributed genetic loci, and 6 of these tumors were examined by Comparative Genome Hybridization. In addition to deletions of the MEN1 gene on Chromosome 11 in two tumors, there were also deletions of large regions of Chromosome 1 in two other tumors. Both of the Chromosome 1 deletions involved a breakpoint near the centromere, a region commonly affected in human malignancies. In addition, aneuploidy was seen in the 4 large or aggressive tumors studied and was not seen in the 4 small indolent tumors. In most cases, this was clearly due to trisomy of the affected chromosome. One tumor showed aneuploidy of 13 different chromosomes, and this tumor also showed microsatellite instability, indicative of a disruption of the mismatch repair pathway. In contrast to many other human tumor types, large scale amplification of any genomic region was absent.